Habitat Relations

Longevity and Reuse of Black Bear Dens inManaged Forests of Coastal British Columbia

HELEN DAVIS,1 Artemis Wildlife Consultants, 4515 Hullcar Road, Armstrong, BC, Canada V0E 1B4

ANTHONY N. HAMILTON, Ministry of Environment, P.O. Box 9338, STN PROV GOVT, Victoria, BC, Canada V8W 9M1

ALTON S. HARESTAD, Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6

RICHARD D. WEIR, Artemis Wildlife Consultants, 4515 Hullcar Road, Armstrong, BC, Canada V0E 1B4

ABSTRACT We evaluated longevity and reuse of denning structures by American black bears (Ursusamericanus) in coastal temperate rainforests of British Columbia from 1992 to 2010 to assess potentialimpacts of forest management on these critical habitat features over time.We identified 67 dens during a 4-yrintensive radio-telemetry study (1992–1995): 40 dens of 21 radio-collared black bears and 27 dens foundincidentally. Dens occurred in or beneath large diameter trees or wooden structures derived from trees (i.e.,logs, root boles, and stumps; x ¼ 143 cm diameter, SD ¼ 49 cm). Longevity of dens varied by type, treespecies that formed the den, and forest management that occurred at or near the den. Twenty-four of 28 densof radio-collared bears that were monitored were still usable in 2010, whereas only 5 of 14 dens foundincidentally were still usable in 2010. We assessed reuse of bear dens 3 times following initial identification:during the radio-telemetry study, again in 2000, and finally in 2010. Radio-collared bears reused dens fromprevious years on 7 of 25 potential occasions during the course of the radio-telemetry study. Upon assessmentin 2000 and 2010, 17 of 24 (71%) available dens first used by radio-collared bears were reused at least oncebetween 1993 and 2010. The high rate of reuse may indicate low availability of den structures in our studyarea. Because black bears in coastal British Columbia only used trees or structures derived from trees forwinter dens and forest harvesting reduces the supply of these necessary structures, conservation andrecruitment of suitable den trees is necessary if maintaining black bear populations is a managementgoal in these areas. � 2011 The Wildlife Society.

KEY WORDS American black bear, British Columbia, dens, forest management, reuse, Ursus americanus.

American black bears (Ursus americanus) hibernate to surviveperiods of low food availability and low temperatures byaccumulating body fat, entering a period of dormancy inwhich metabolic expenditures are lowered, and drawing onthese stored energy reserves. Consequently, winter hiberna-tion is a vital aspect of the life history of bears and selection ofwinter den sites is important because dens meet several liferequisites during the over-wintering period. Specifically,dens must confer thermal benefits, protect bears from in-clement weather, and provide security from disturbance andpredation (Johnson and Pelton 1981).A wide range in frequency of reuse of winter dens by black

bears has been reported. In various studies, reuse has rangedfrom nonexistent (Tietje and Ruff 1980, Kolenosky andStrathearn 1987), to low (15%; Crook and Chamberlain2010) to high (47–71%; Schwartz et al. 1987). Lindzeyand Meslow (1976) found that at least 6 of 12 dens of blackbears were reused in an area of Washington with extensiveforest harvesting.

Bears may occasionally use previously occupied dens for avariety of reasons. In areas with few denning opportunities,bears have few sites from which to select and therefore mayreuse dens frequently. Many researchers have concluded thatthe rate of reuse may be linked to the supply of potential densites. Several researchers have suggested that high rates ofreuse may indicate low availability of suitable den sites(Johnson 1978, Beecham et al. 1983, LeCount 1983, Altand Gruttadauria 1984). Alternatively, bears may reuse densfrom year-to-year because a site was successfully used during1 over-wintering period, so use continues in subsequentperiods.In coastal temperate rainforests of western North America,

black bears hibernate through cool, wet winters that presentdenning animals with considerable environmental challengesthat limit the range of structures suitable for denning. Thecombination of persistent rainfall, lack of permanent snowcover, and cool temperatures affects the types of structuresthat provide shelter from these abiotic factors. Because blackbears in coastal British Columbia rely heavily upon largediameter trees or wooden structures derived from trees (i.e.,logs, root boles, and stumps; Noble et al. 1990, Davis 1996),the broad-scale harvest of late-successional forests may di-minish the supply of dens for black bears. Our objectives were

Received: 29 March 2011; Accepted: 22 July 2011;Published: 18 October 2011

1E-mail: hdavis@artemiswildlife.com

The Journal of Wildlife Management 76(3):523–527; 2012; DOI: 10.1002/jwmg.253

Davis et al. � Longevity and Reuse of Black Bear Dens 523

to 1) report reuse of black bear dens to determine whetherspecial management actions were necessary to preserve anadequate supply of dens for coastal black bear populations,and 2) investigate the efficacy of past management actions atretaining existing dens.

STUDY AREA

Our 20,000-ha study area was located in the NimpkishValley (508 140, 1268 520), 40 km south of Port McNeillon northern Vancouver Island, British Columbia, Canada.The study area was comprised of the Coastal WesternHemlock and Mountain Hemlock biogeoclimatic zones(Nuszdorfer 1991), with elevations ranging from 20 m to>1,500 m. Average annual precipitation was 1,780 mm,with 4–5% of that falling as snow. Dominant species ofconiferous trees includedDouglas-fir (Pseudotsuga menziesii),western hemlock (Tsuga heterophylla), mountain hemlock(T. mertensiana), western redcedar (Thuja plicata), yellow-cedar (Chamaecyparis nootkatensis), Pacific silver fir (Abiesamabilis), and Sitka spruce (Picea sitchensis). Deciduoustree species were rare except in riparian zones.Our study area had a long history of commercial forest

harvesting typical of coastal temperate rainforests in BritishColumbia. Forests in the area had been harvested by clear-cutting since the 1920s and most clearcuts regenerated toconifer-dominated, structurally and floristically depauperate,second-growth stands with closed canopies. Approximately45% of the forests in the study area were exposed to distur-bance (primarily forest harvesting) between 1923 and 1995(Green 2000). Most harvesting occurred in old-growth for-ests but harvesting of second-growth forests began in the2000s. When forest harvesting began in the valley, tallstumps and large pieces of coarse woody debris were leftin clearcuts because of lower utilization standards than pres-ent day.

METHODS

We identified winter dens of black bears using 2 methods.Between 1992 and 1995, we followed radio-collared blackbears to their dens (Davis 1996). We also found other densincidentally while collecting field data and forestry crewsfound dens while surveying proposed cutblocks duringboth denning and non-denning seasons. We initially ra-dio-collared bears in 1992, so dens for each individualwere first identified during the winter of 1992–1993.Thus, the first opportunity to observe reuse occurred duringthe winter of 1993–1994. For each den, we recorded dentype, species and diameter of tree used, the amount andspecies of vegetation used as bedding material inside theden, and searched the immediate area for fecal plugs(Hamilton and Marchinton 1980). We photographeddens and den bedding to allow for comparisons among years.We used several methods to assess reuse of known dens by

black bears. During visits to dens of radio-collared bears inthe spring, we noted that recently used dens contained greenvegetation that the bears carried into the den for bedding.We used this obvious sign as the primary criteria to evaluaterecent reuse in subsequent visits. We also considered reuse to

have occurred if additional bedding had been added ormodifications were made since we last visited the den.Furthermore, we looked for recent fecal plugs near theden and the presence of spider-webs and fungus growingwithin cavities to determine whether use was recent or not.Our estimates of reuse may be conservative because we wereprobably only able to conclusively detect reuse during theprevious 2–3 yr.We monitored each den structure for reuse by black bears

during 3 periods. During 1993–1995, we tracked radio-collared bears to dens. Radio-collared bears were monitoredfor up to 3 consecutive winters, which allowed us to estimatean intensive, annual rate of reuse of dens by these bears. Wethen assessed a sample of known dens again in 2000 and in2010. The same observer visited the dens during each as-sessment period to reduce observer bias. These subsequentvisits allowed us to assess whether dens were reused at leastonce during the intervening period.Wewere unable to revisitall dens because of closed roads and funding constraints; wedid not continue to assess dens that were decayed ordestroyed.We also examined the efficacy of several forest manage-

ment prescriptions for conserving den structures. With thecooperation of forest managers, several dens identified dur-ing our radiotelemetry study were retained in or aroundproposed forest harvest units. Dens in hollow trees wereretained as single trees within clearcuts, within retentionpatches in clearcuts, or on edges of clearcuts through bound-ary modifications. Some dens in logs were also retained inclearcuts. Prescriptions were typically applied in old-growthand second-growth stands that were scheduled for clearcutharvesting. We visited these dens to determine if they werestill usable (i.e., structurally intact) in 2000 and 2010.

RESULTS

We identified 40 unique dens of 21 radio-collared black bearsand incidentally located 27 dens of unmarked black bearsduring the 3 winters of 1992–1993, 1993–1994, and 1994–1995. Black bears used dens in or beneath large diametertrees or wooden structures derived from trees (i.e., logs, rootboles, and stumps; x ¼ 143 cm diameter, SD ¼ 49 cm).Wecategorized dens found between 1992 and 1995 into 5 types(Davis 1996): in live or dead hollow trees (n ¼ 37, 55%),under the root masses of overturned trees (root bole, n ¼ 15,22%), in the base of high-cut tree stumps (n ¼ 8, 12%),inside or under large logs (n ¼ 4, 6%) or amongst the roots ofstanding live trees (n ¼ 3, 4%).During the radio-telemetry monitoring period, we had the

potential to observe 25 incidents of reuse (i.e., the bear wasmonitored during a denning period subsequent to its first-detected den period). Radio-collared black bears reused denson 7 occasions (28%). The proportions of reuse during theradio-telemetry study were not noticeably different betweenmale and female bears (4 cases of reuse by 3 males, 3 casesof reuse by 3 females). Five bears used the same den twice(2 males, 3 females) and 1 adult male used the same den for3 consecutive winters. We did not observe any radio-collared

524 The Journal of Wildlife Management � 76(3)

bears reusing dens of other radio-collared bears during ourradio-telemetry monitoring period.We revisited known dens during 2000 and 2010 to verify

whether they were still intact and useable and to evaluatereuse by black bears since initial den identification in the1990s (Table 1). We assessed 28 dens of radio-collared bearsduring the 2 visits, 22 of which we visited in both 2000 and2010. Additionally, we assessed 14 incidentally found dens in2000 and 2010; we visited 8 in both years, whereas 6 wereonly visited once. Many dens showed conspicuous signs ofreuse, with obvious recent, green bedding and several hadclearly been reused the previous winter. In other cases,although reuse had occurred since the previous assessment,it was not possible to determine the year in which the den hadbeen reused. Use of den sites among years was not limited toa single bear; we documented dens being used after theknown occupant had perished.Most dens of radio-collared bears and incidentally located

dens were reused at some time between 1993 and 2010, with17 of 24 (71%) dens of radio-collared bears reused during thisperiod (Table 1). Similarly, 3 of 5 incidentally located denswere reused between 1993 and 2010; only 5 of 14 incidentallylocated dens were available for reuse by 2010 because 9 denshad either decayed, been cut down during forest harvesting,or had blown down after being retained during forestharvesting.Longevity of den structures used by radio-collared bears

varied by den type and tree species (Table 1). Of the 9 rootbole structures that were used as dens between 1992 and1995, 6 remained after 18 yr. Three were decayed ordestroyed in second-growth harvesting, and thus no longeravailable for bears. However, only 3 of the 9 root bole denswere still well-suited for bear dens as of 2010; 3 were judgedto be of low quality due to decay since use was first docu-mented. In contrast, all 12 dens in hollow western redcedarand yellow-cedar trees, and 3 dens in stumps (2 Sitka spruceand 1 western hemlock) showed no significant signs of decaythat made them unusable. Dens in standing trees remainedusable through the entire assessment period, unless they were

cut down during forest harvesting operations or blew downafter retention as single trees. Of the 19 dens in or understanding trees, 5 were still usable as dens in old-growthforests as of 2010, 7 were retained by modifying forest harvestplans, 1 was felled and 6 could not be visited because ofremoteness but were not known to be within harvested areas.Dens that occurred in second-growth forests showed con-

siderable decline in suitability over the assessment period(Table 1). Only 7 of 14 dens (50%) of radio-collared bearsthat occurred in second-growth forests in 1995 were fullyintact and useable in 2010. Of the other 7, 4 were of muchdecreased quality (i.e., decay had increased the entrance size)but could possibly still be suitable, 2 were too decayed for use,and 1 was destroyed during second-growth harvesting.The frequency of reuse varied considerably among different

types of den structures used by radio-collared bears (Table 1).All 3 dens in large high-cut stumps showed evidence ofrecent reuse in 2000 and 2010. We assessed 3 log dens in2000, all of which had recently been reused; however, by2010, 1 den under a Douglas-fir log had decayed consider-ably and was marginally useable. Eight of 12 hollow tree denswere reused at some time between 1993 and 2010. However,we observed considerably less reuse of root bole dens: 3 of9 root bole dens had been reused (3 were too decayed for use).We were unable to assess reuse of any dens located undertrees because of their remote locations.Reuse of dens by radio-collared bears also varied by distur-

bance history of the stand in which they occurred. Althoughreuse was high across the landscape, reuse was greater insecond-growth stands (9 of 11 dens reused, 82%) than in oldgrowth stands (8 of 13 dens reused, 62%). Four of the 8reused dens that were within old growth forests when firstused continued to be used after they were retained withinclearcuts.We investigated the efficacy of forest management pre-

scriptions to conserve 15 dens between 1993 and 2010. Oneden that was found incidentally was not suitable for retentionas a standing tree because of safety concerns, it was cut so thata stump approximately 2 m in height remained. This den had

Table 1. Number of dens suitable for reuse and frequency of den reuse by American black bears in the Nimpkish Valley, British Columbia, Canada between1993 and 2010. Not all dens that were identified could be assessed for reuse during each assessment period.

Den typeNo.

revisited

Old-growth dens Second-growth dens

Totalavailable

forreuse

Reused�1 time

Intact inold-growth

Retained inharvestedareasa

Cut downin old-growthharvesting

Intact insecond-growth

Of decreasedquality

Decayed,not

reuseable

Destroyedduring

second-growthharvesting

Dens of radio-collared bearsHollow tree 13 6 6 1 12 8Log 3 2 1 3 3Root bole 9 1 2 3 2 1 6 3Stump 3 3 3 3Total 28 7 6 1 7 4 2 1 24 17

Incidentally located densHollow tree 12 2 6b 4 5 3Log 1 1Root bole 1 1Total 14 2 7 4 1 5 3

a Retained as a single tree, retention patch, or on edge of clearcut.b Three trees blew down of the 6 retained.

Davis et al. � Longevity and Reuse of Black Bear Dens 525

not been reused when checked in 2000 and 2010. One den ina Douglas-fir log was retained in a clearcut but it wassubsequently destroyed by a bear foraging for insects. Aden in a hollow tree used by a male radio-collared bearwas preserved through boundary modifications so that theden remained 30 m inside contiguous old-growth forest. Aroad was constructed approximately 50 m below the den thesummer after we first documented use of the den; despitethis, the bear reused the den for a second winter. Prior to thethird winter of consecutive use, a clearcut was harvested 30 maway. The den was reused again between 1995 and 2000. Sixdens in hollow trees were left as single trees within clearcuts;2 of these had blown down by 2010. Five dens in hollow treeswere retained as wildlife tree patches; none of these dens hadblown down by 2010, although 1 was only checked in 2000and could not be surveyed in 2010. One hollow tree den lefton the boundary of a cutblock had also blown down by 2000.This den, and 1 other den in a hollow tree within thecutblock that also blew down, may be suitable for use bybears in the future as log dens.

DISCUSSION

Black bears in coastal environments prefer hollow trees fordenning, likely for the thermal and security benefits thatthese structures confer relative to other structures (Johnsonet al. 1978, Noble et al. 1990, Davis 1996), but large hollowtrees are generally removed during forest harvesting oper-ations. Hence, denning opportunities are likely substantiallylesser in areas with extensive forest harvesting compared toless disturbed landscapes (Johnson and Pelton 1981).Depending on the forest utilization standards, few suitablestructures (e.g., logs and high stumps) remain after forestharvesting, and these are likely much rarer inmanaged foreststhan hollow trees in old-growth forests. Thus, in second-growth forests, bears likely have a narrower range of struc-tures in which to den and this may be reflected in a high rateof reuse of the few remaining structures.The different types of structures used by bears for denning

appeared to have different longevity rates, which likely fur-ther affected the rate of den reuse that we observed. Denstructures can only be reused if they remain structurallysound. For example, dens excavated into the ground andbrush piles may only last 1 or 2 yr before collapsing, whereasrock cavities may persist for many years (Alt andGruttadauria 1984, Beck 1991). Longevity of tree dens inour study depended upon tree species and the type of denstructure. Dens comprised of western redcedar may persistfor hundreds of years because of this species’ relatively highresistance to decay (Minore 1983). Living hollow treesappeared to have the highest longevity rate of den structuresthat we observed, and thus are the most likely to be availablefor longer durations than other structures. However, largeold trees are also those most likely to be removed duringforest harvesting. Because of their growth and decay char-acteristics, dens in hollow cedar trees may be very importantstructural legacies that should be protected during harvestingof future forest rotations.

In the temperate rainforests of coastal British Columbia,we found considerable reuse of den structures by black bearsamong years by the same bear and even among bears. Thefrequency of repeated use of dens during the telemetrystudy (28%; Davis 1996) was relatively high compared tothat reported in several studies (e.g., 4.8%, Pennsylvania,Alt and Gruttadauria 1984; 15%, Louisiana, Crook andChamberlain 2010) but not others. Over our longer periodof monitoring, the high rate of 71% reuse of dens over the>15 yr was greater than that found in other studies in coastaltemperate forests with extensive forest harvesting (e.g.,�50%, coastal Washington, Lindzey and Meslow 1976),although their monitoring period was shorter.Black bears may not reuse dens every year for a number of

reasons. First, dens may not be reused because predators,such as other bears (Alt and Gruttadauria 1984, Davis andHarestad 1996) or wolves (Canis lupus; Horejsi et al. 1984),may repeatedly check low-security dens for occupants. Infact, 1 of only 2 dens not reused in second-growth forests wasa root bole den at which a female and cubs were killed byanother bear (Davis and Harestad 1996). Second, repeateduse may increase the load of ectoparasites in the den struc-ture, as suspected by Beck (1991). We encountered fleas(Siphonaptera: Vermipsyllidae Chaetopsylla spp.) in numer-ous black bear dens in the study area, although the presenceand longevity of fleas was not assessed directly. Fleas that wecollected were alive several months after bears had left theirdens and may be able to remain dormant in dens for longperiods, supporting the hypothesis that ectoparasite load mayaffect reuse. Other species of fleas have been found to rest forlong periods as larva (up to 600 days; Bacot 1914). For thisreason, it may be important to quantify reuse over longerperiods of times; den structures may need to be vacant forseveral years for the ectoparasite load to diminish.The high rate of den reuse that we observed may be linked

to either the low supply of these structures, the benefits thatknown dens confer upon the individual, or both. Bears mayreuse a specific den in consecutive years because the benefitsof the den are known to the bear and the individual does notrisk using an unknown den site. However, the most likelyexplanation is that the availability of den sites is constrainedin the Nimpkish Valley. In portions of the study area withextensive second-growth forests, 9 of 11 (82%) dens werereused at least once, with several being used at least 4 times.This suggests that den structures are limited in these struc-tural stages; high reuse may indicate low availability ofsuitable alternatives (Beecham et al. 1983, LeCount 1983,Alt and Gruttadauria 1984). However, although reuse ratesin old growth were somewhat lesser than that in second-growth forests, the rate was still high (62%) which suggeststhat the overall landscape supply of dens may be diminished.

MANAGEMENT IMPLICATIONS

Conversion of late-successional forests to younger even-agedstands is detrimental to the supply of dens for black bears incoastal British Columbia. Conservation of adequate dens anddenning habitat in forest management plans is appropriatebecause individual den sites are important structural legacies

526 The Journal of Wildlife Management � 76(3)

to black bear populations and a lack of suitable den sites maylead to decreased black bear populations because of increasedcannibalism (Alt and Gruttadauria 1984, Davis andHarestad 1996), predation by wolves (Horejsi et al. 1984),or deaths due to increased energetic costs from using lessthermally advantageous dens (e.g., dens prone to inundation;Alt 1984). If maintaining current population levels is amanagement goal, then conservation and recruitment ofadequate numbers of safe winter den sites across the land-scape needs to be included in forest management plans.Our findings suggest that the most effective method forretaining highly persistent, hollow tree dens is to retain theseimportant structures within contiguous forests or patches oftrees rather than as single trees. In other forest types,researchers have found that retention patches >1 ha insize, placed in topographically sheltered locations, with min-imized edge exposure to prevailing winds, are more windfirm(Steventon 2011). Further research on the longevity of wild-life tree patches is required in coastal forests to determine themost effective management prescriptions for long-term re-tention of individual den trees. However, managing habitatfor black bears must include not only retention of current denstructures but also recruitment of new den structures and theecological processes that create those structures.

ACKNOWLEDGMENTS

Funding for our research was provided by the HabitatSilviculture Protection Account, Habitat ConservationTrust Fund, and Habitat Conservation Trust Foundation,Canadian Forest Products Ltd. and Western ForestProducts, Inc., Wildlife Habitat Canada, and GlobalForest (GF-18-1999-23). We thank J. Deal and CanadianForest Products Ltd. for initiating the project and voluntarilyimplementing management prescriptions to retain den struc-tures. We are indebted to A. Hahn, A. Friedman, P.Kaczensky, M. Kellner, C. Mueller, R. Ramcharita, andD. Wellwood for their assistance in the field and T.Poland for insect identification.

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Associate Editor: Michael Chamberlain.

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